Re: [Discuss-gnuradio] external clock

[Robert McGwier]

Bdale:

I am using an atom bomb to smash an ant:

I have a PTS-160 synthesizer.  It is locked with a rubidium oscillator 
OR a GPS tamed oscillator.  I use the rubidium when I am moving (at 
W3CCX or in the car or on 10 Ghz cumulative weekends, etc.)  Moving GPS 
sucks for taming an oscillator!   I have not yet had time to do all of 
the necessary evaluation of these approaches.  I feed the output to a 
distro box that has a few outputs and one CAN be the USRP.  I have not 
yet done a thing beyond proving it works.  The PTS-160 is a marvel and 
it has very good phase noise at these frequencies (64 Mhz).  With this 
lashup, I do not have to use the PLL.

PLL's are great, so far as they go.  We would have to know a lot more 
about this particular one before we could say if your idea is a good one 
or a bad one.  How do we ask the right questions?  We need to know what 
the transfer function of the loop filter in the PLL.   What you are 
proposing is commonly used to frequency lock an oscillator but 
introduces more phase noise (MUCH MORE) than the crystal you are trying 
to replace. If your system has a certain phase noise at 10 Mhz,  you can 
be certain that it will have many dB more at 64 Mhz. The very best 
approach for what you are asking is to find a 64 Mhz TC-VCXO and to use 
your GPS in a system to pull the TC-VCXO on frequency since you are 
going to have one heck of a time beating the phase noise of the crystal 
oscillator in TC-VCXO.  Phase noise will be a killer deal in your EME 
applications as you know.  You can take TAPR's new Reflock II and tame 
the oscillator frequency.  Valpey-Fischer makes such an oscillator but 
they do not sell one and do not sample.  We would need to come up with a 
description of what it is we would want to do,  get the Reflock II in 
our hands,  tame the TC-VCXO with GPS and then suggest a group buy on 
these oscillators which will very likely be the best way to go.  I have 
an interest in locking 2 then 4 then  . . . USRP's.  This will be to 
build a phased array.  I also need to figure out to synchronize the 
blocks at the sample "name".  One step at a time  . . .



Bob


Bdale Garbee wrote:

> Looking at options for locking all the clocks that are part of the EME
> station I'm building to my HP 58503A GPS-synced clock.  Searching the
> list archives, I found this note from Matt regarding the external clock
> input on the USRP:
>
> http://lists.gnu.org/archive/html/discuss-gnuradio/2005-03/msg00030.html
>
> Has anyone played with this?  
>
> Of particular interest is whether anyone has hacked the FPGA code to use
> the internal PLLs to retain a 64 Mhz sample clock using a 10 Mhz
> reference clock input to the board.
>
> Bdale
>
>
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